(Parts of this background may or may not constitute prior art.) Conventional optical fiber cables for indoor use typically provide a convenient termination for standard single-fiber connectors, such as ST, SC or LC connectors, often using tight buffered optical fiber with an outer diameter of 900 microns. However, multifiber connectors are becoming increasingly popular in order to save space and installation labor. These connectors use multi-fiber “MT” ferrules. 12-fiber multifiber connectors with a “MT” type ferrule can be used for connection of twelve 250 micron fibers in the same space normally needed for 2 traditional SC connections, or 3 traditional LC connections. Commercially available multifiber connectors include MTP® connectors from US Conec, and MPO connectors from Furukawa America or Tyco Electronics.
These types of multifiber connectors are designed to work with flat optical ribbons. However, use of flat ribbons in cable may lead to undesirable cable performance in the field, e.g., difficult cable handling and routing in the field. Flat cables are prone to twisting and kinking. If, on the other hand, a flat ribbon is placed in a round cable, the cable must be fairly large and bulky in order to fit the flat ribbon within a robust round structure. For example, a 12-fiber ribbon, made using 250 micron fibers, is typically 3.1 mm wide; placing jacketing and reinforcement over that ribbon leads to a round cable in excess of 5 mm in diameter: an undesirably large cable.
To address these problems with ribbon cable, some providers of multifiber connectors offer compact, round, indoor optical cables using unribboned, colored, loose, 250 micron fiber. Colored 250 micron fiber resembles the type of fiber often used in outside plant cables. The individual 250 micron fibers can be packed very tightly into a profile that is substantially round, thus allowing packaging those fibers in a small round cable.
Commercial examples of this sort of cable include the “Premise MicroCore” cable, by AFL Telecommunications and Corning “MIC250” cables. The AFL 12-fiber cable is 4.5 mm in diameter; the Corning cable is 4.4 mm in diameter. Both of these cables can be used as subunits for higher fiber count cables; the AFL design may have as many as 72 fibers, while the Corning design is offered with 24 fibers.
However, multifiber connectors that use MT ferrules are designed to accept flat ribbons, so special accommodations are made for round, loose fiber cables with multifiber connectors. For example, the loose fiber may be ‘ribbonized’ prior to use with MT-type multifiber ferrules. Commercial kits for ribbonization are available from, for example, US Conec. In factory ribbonization, the individual fibers may be broken out from the end of the small, round cable, and formed into a short ‘ribbon’ using either a UV-cured resin or engineered adhesive tapes. After the fibers are ribbonized, they may be terminated with the multifiber connector. This approach requires extra time in connectorization, but provides a terminated multifiber jumper with reduced size and improved handling for field installation.
However, the round cable designs just described have several drawbacks:                1. Poor fiber management. The colored, 250 micron fibers are loosely laid inside the cable with aramid yarn reinforcement. When the cable jacket is opened, the fibers are randomly organized, and randomly mixed with strands of aramid yarn. In the ribbonizing process, the operator cuts or folds back the aramid yarn to expose the fiber, then picks out the fibers in the order required for ribbonizing. This is a tedious process. In addition, the fibers are free to twist, and change locations, when the cable is stretched, bent, etc.        2. Poor fiber protection. The fibers are prone to being damaged during the ribbonizing process. In these cable designs there is little mechanical protection for the fibers when the cable is opened, and the operator must take extreme care to ensure no fibers are damaged when the aramid yarn is removed and the fibers are ordered one-by-one for ribbonizing.        3. Poor crush protection. The hollow core and bare-fiber structure of these cables means that crushing loads may be translated directly to the fibers. When crushed, the fibers may be pressed one against another. Moreover, the organization of the fibers relative to each other can be rearranged. These effects may result in high point attenuation and/or broken fibers, and limits the suitability of these cables for many indoor applications. While these cables may be adequate for frame-to-frame interconnect applications, where they are installed in a relatively benign environment, they may not be sufficiently robust for installation in overhead or under-floor ladder racks, or raceways for room-to-room connections.        